Fix a bug in the ap function

This could prevent it from updating values if you returned to
the original value. E.g., if your values went 1, 2, 1, 2, you'd
get notifications for 1, 2, 2. This would also cause some extra
notifications to go when the logical value didn't actually change.

src/main/scala/org/gerweck/scalafx/util/observable.scala

@@ -42,6 +42,7 @@ trait ObservableImplicits {
         val newVal = recalculate()
         if (prevValue != newVal) {
           prop.value = newVal
+          prevValue = newVal
         }
       }